1. Field of the Invention
The present invention relates to a lithographic apparatus, an array of individually controllable elements and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
One type of known array of individually controllable elements include arrays of small mirrors. Each of the mirrors is arranged such that it can rotate about a hinge and is associated with an actuator for pivoting the mirror. A lithographic apparatus utilizing such an array of individually controllable elements can be configured such that when each mirror is in a first position, radiation impinging upon it from an illumination system is reflected into an aperture of the projection system. When the mirror is in a second position, the radiation is reflected away from the aperture of the projection system. Accordingly, by setting some mirrors to the first position and some mirrors to the second position, it is possible to modulate the beam of radiation projected onto a substrate by the projection system, thereby enabling the exposure of a pattern on the substrate.
It has further been proposed to configure the array of individually controllable elements such that the mirror can be set to intermediate positions between the first and second positions. At such intermediate positions the radiation directed to the aperture of the projection system by each mirror can be of a correspondingly intermediate radiation intensity. Accordingly, it is possible to project onto the substrate a pattern of radiation that includes gray tones and/or it is possible to use grayscale radiation intensity control in order to finely adjust the position of the boundaries of pattern features formed on the substrate.
However, known arrays of individually controllable elements use electrostatic actuators in order to control the position of the mirrors. Such actuators are adequate for controlling the position of the mirrors between first and second positions, corresponding to substantially all incident radiation or substantially no radiation being directed to the aperture of the projection system. This is because in such arrangements the precise position of the mirror is not critical. However, where intermediate mirror positions are to be used in order to provide grayscale control, the precise position of the mirror is important because it determines the intensity of the radiation directed into the projection system. Furthermore, the relationship between the voltage applied to the electrostatic actuator and the displacement of the mirror is non-linear. Accordingly, the control system is necessarily complex because for each mirror, a required corresponding intensity of radiation at the substrate will need to be converted to a required voltage, necessary to generate a given displacement of the mirror that directs a given intensity of radiation into the projection system. As a result of the non-linear relationship between the voltage and the displacement of the mirror, this determination is computationally complex or may be performed by lookup tables, thus requiring a significant amount of memory. This can be especially significant because each mirror can respond differently and therefore can require its own lookup table. Furthermore, the configuration can be unstable and, above a given voltage applied to the actuator or a given displacement of the mirror, the mirror will snap through.
Therefore, what is needed is a system and method for controlling arrays of individually controllable elements.